The heater radiator in the MBs was I believe uprated from those in the 6's - certainly our MB heater is very good whereas our 6A heater was very bad. Try getting a MB standard heater rad and you will find its more than up to the job.

Transistors tend to either work or not work - knowing that you have had rough running for a while \i would be surprised if is duff. The pcb in the dizzy however is not so reliable - I have had one fail and so has Mark Wilson - do you know anyone who can lend you one to try.